Fix dashboard UI bugs and more (#14767)

This PR fixes a few UI bugs I spontaneously encountered:

- Fixes emojis in repo titles getting head-cut and tail-cut in dashboard feed due to introduction of 1.25 em emojis at 1 em line-height, by simply using the original 1 3/7 em value of `semantic.css`
- Fixes regression (too long repo names should be capped to 70%) in #13828 due to flex children not respecting properties like `overflow: hidden;`, and removes a block of dead style code
- Follow-up to #14761, removes extraneous code for top navbar and correct right margin for Font Awesome
- Fixes color emphasis inversion in arc-green theme for top right buttons (edit, delete) on commit view boxes

web_src/less/_repository.less

@@ -383,7 +383,7 @@
             padding: 5px;
             margin-left: 5px;
             line-height: 1;
-            color: #767676;
+            color: var(--color-text);
             vertical-align: middle;
             background: transparent;
             border: 0;
@@ -399,7 +399,8 @@
           }
 
           .btn-octicon.disabled {
-            color: #bbbbbb;
+            color: inherit;
+            opacity: var(--opacity-disabled);
             cursor: default;
           }