Remove EasyMDE focus outline on text (#25328) (#25332)

Backport #25328 by @silverwind

EasyMDE in Firefox currently shows a ugly outline in the fake textarea
the CodeMirror uses. Hide it.

Before:

<img width="845" alt="Screenshot 2023-06-18 at 02 54 09"
src="https://github.com/go-gitea/gitea/assets/115237/dc406166-9ad5-4a9b-9581-002b5cdcc6df">

After:

<img width="870" alt="Screenshot 2023-06-18 at 02 54 24"
src="https://github.com/go-gitea/gitea/assets/115237/ddd78759-2cf2-4385-b863-7576fec25c34">

Co-authored-by: silverwind <me@silverwind.io>

web_src/css/codemirror/base.css

@@ -43,3 +43,7 @@
 .CodeMirror-focused {
   border-color: var(--color-primary) !important;
 }
+
+.CodeMirror :focus {
+  outline: none;
+}