Add issue number to the search index, rank number and title matches higher (#7956) (#7968)

An attempt at solving #7956. This (and rebuilding the index) seems enough to ensure the issue *appears* among the results.

However, I couldn't figure out from [bleve docs](https://github.com/blevesearch/bleve/blob/master/docs/scoring.md) how to affect the scoring based on specific fields, or whether that is possible at all.

Disclaimer: I've never written Go before, sorry 😅 take it as a quick PoC more than anything.

### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] I do not want this change to show in the release notes.
- [x] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

<!--start release-notes-assistant-->

## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Features
  - [PR](https://codeberg.org/forgejo/forgejo/pulls/7968): <!--number 7968 --><!--line 0 --><!--description QWRkIGlzc3VlIG51bWJlciB0byB0aGUgc2VhcmNoIGluZGV4LCByYW5rIG51bWJlciBhbmQgdGl0bGUgbWF0Y2hlcyBoaWdoZXIgKCM3OTU2KQ==-->Add issue number to the search index, rank number and title matches higher (#7956)<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7968
Reviewed-by: Shiny Nematoda <snematoda@noreply.codeberg.org>
Co-authored-by: Danko Aleksejevs <danko@very.lv>
Co-committed-by: Danko Aleksejevs <danko@very.lv>

modules/indexer/issues/internal/model.go

@@ -14,6 +14,7 @@ type IndexerData struct {
 	ID       int64 `json:"id"`
 	RepoID   int64 `json:"repo_id"`
 	IsPublic bool  `json:"is_public"` // If the repo is public
+	Index    int64 `json:"index"`
 
 	// Fields used for keyword searching
 	Title    string   `json:"title"`