Use frontend fetch for branch dropdown component (#25719)

- Send request to get branch/tag list, use loading icon when waiting for
response.
- Only fetch when the first time branch/tag list shows.
- For backend, removed assignment to `ctx.Data["Branches"]` and
`ctx.Data["Tags"]` from `context/repo.go` and passed these data wherever
needed.
- Changed some `v-if` to `v-show` and used native `svg` as mentioned in
https://github.com/go-gitea/gitea/pull/25719#issuecomment-1631712757 to
improve perfomance when there are a lot of branches.
- Places Used the dropdown component:

     Repo Home Page
    
<img width="1429" alt="Screen Shot 2023-07-06 at 12 17 51"
src="6accc7b6-8d37-4e88-ae1a-bd2b3b927ea0">

    Commits Page

<img width="1431" alt="Screen Shot 2023-07-06 at 12 18 34"
src="2d0bf306-d1e2-45a8-a784-bc424879f537">

    Specific commit -> operations -> cherry-pick
    
<img width="758" alt="Screen Shot 2023-07-06 at 12 23 28"
src="1e557948-3881-4e45-a625-8ef36d45ae2d">

    Release Page
    
<img width="1433" alt="Screen Shot 2023-07-06 at 12 25 05"
src="3ec82af1-15a4-4162-a50b-04a9502161bb">

- Demo


d45d266b-3eb0-465a-82f9-57f78dc5f9f3

- Note:

UI of dropdown menu could be improved in another PR as it should apply
to more dropdown menus.

Fix #14180

---------

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

routers/web/repo/compare.go

@@ -754,6 +754,12 @@ func CompareDiff(ctx *context.Context) {
 	}
 	ctx.Data["HeadBranches"] = headBranches
 
+	// For compare repo branches
+	PrepareBranchList(ctx)
+	if ctx.Written() {
+		return
+	}
+
 	headTags, err := repo_model.GetTagNamesByRepoID(ctx, ci.HeadRepo.ID)
 	if err != nil {
 		ctx.ServerError("GetTagNamesByRepoID", err)