Add codespell support and fix a good number of typos with its help (#3270)

More about codespell: https://github.com/codespell-project/codespell .

I personally introduced it to dozens if not hundreds of projects already and so far only positive feedback.

```
❯ grep lint-spell Makefile
	@echo " - lint-spell                       lint spelling"
	@echo " - lint-spell-fix                   lint spelling and fix issues"
lint: lint-frontend lint-backend lint-spell
lint-fix: lint-frontend-fix lint-backend-fix lint-spell-fix
.PHONY: lint-spell
lint-spell: lint-codespell
.PHONY: lint-spell-fix
lint-spell-fix: lint-codespell-fix
❯ git grep lint- -- .forgejo/
.forgejo/workflows/testing.yml:      - run: make --always-make -j$(nproc) lint-backend checks-backend # ensure the "go-licenses" make target runs
.forgejo/workflows/testing.yml:      - run: make lint-frontend
```
so how would you like me to invoke `lint-codespell` on CI? (without that would be IMHO very suboptimal and let typos sneak in)

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/3270
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Yaroslav Halchenko <debian@onerussian.com>
Co-committed-by: Yaroslav Halchenko <debian@onerussian.com>

models/fixtures/TestGetUnmergedPullRequestsByHeadInfoMax/pull_request.yml

@@ -1,7 +1,7 @@
 -
   id: 1001
   type: 0 # pull request
-  status: 2 # mergable
+  status: 2 # mergeable
   issue_id: 1001
   index: 1001
   head_repo_id: 1

models/fixtures/commit_status.yml

@@ -15,7 +15,7 @@
   repo_id: 1
   state: "warning"
   sha: "1234123412341234123412341234123412341234"
-  target_url: https://example.com/converage/
+  target_url: https://example.com/coverage/
   description: My awesome Coverage service
   context: cov/awesomeness
   creator_id: 2
@@ -26,7 +26,7 @@
   repo_id: 1
   state: "success"
   sha: "1234123412341234123412341234123412341234"
-  target_url: https://example.com/converage/
+  target_url: https://example.com/coverage/
   description: My awesome Coverage service
   context: cov/awesomeness
   creator_id: 2

models/fixtures/pull_request.yml

@@ -1,7 +1,7 @@
 -
   id: 1
   type: 0 # gitea pull request
-  status: 2 # mergable
+  status: 2 # mergeable
   issue_id: 2
   index: 2
   head_repo_id: 1
@@ -16,7 +16,7 @@
 -
   id: 2
   type: 0 # gitea pull request
-  status: 2 # mergable
+  status: 2 # mergeable
   issue_id: 3
   index: 3
   head_repo_id: 1
@@ -29,7 +29,7 @@
 -
   id: 3
   type: 0 # gitea pull request
-  status: 2 # mergable
+  status: 2 # mergeable
   issue_id: 8
   index: 1
   head_repo_id: 11
@@ -42,7 +42,7 @@
 -
   id: 4
   type: 0 # gitea pull request
-  status: 2 # mergable
+  status: 2 # mergeable
   issue_id: 9
   index: 1
   head_repo_id: 48
@@ -55,7 +55,7 @@
 -
   id: 5 # this PR is outdated (one commit behind branch1 )
   type: 0 # gitea pull request
-  status: 2 # mergable
+  status: 2 # mergeable
   issue_id: 11
   index: 5
   head_repo_id: 1
@@ -68,7 +68,7 @@
 -
   id: 6
   type: 0 # gitea pull request
-  status: 2 # mergable
+  status: 2 # mergeable
   issue_id: 12
   index: 2
   head_repo_id: 3
@@ -81,7 +81,7 @@
 -
   id: 7
   type: 0 # gitea pull request
-  status: 2 # mergable
+  status: 2 # mergeable
   issue_id: 19
   index: 1
   head_repo_id: 58
@@ -94,7 +94,7 @@
 -
   id: 8
   type: 0 # gitea pull request
-  status: 2 # mergable
+  status: 2 # mergeable
   issue_id: 20
   index: 1
   head_repo_id: 23
@@ -103,7 +103,7 @@
 -
   id: 9
   type: 0 # gitea pull request
-  status: 2 # mergable
+  status: 2 # mergeable
   issue_id: 21
   index: 1
   head_repo_id: 60
@@ -112,7 +112,7 @@
 -
   id: 10
   type: 0 # gitea pull request
-  status: 2 # mergable
+  status: 2 # mergeable
   issue_id: 22
   index: 1
   head_repo_id: 61