Remove jQuery AJAX from the repo editor (#29636)

# Preview Tab
- Removed the jQuery AJAX call and replaced with our fetch wrapper
- Tested the preview tab functionality and it works as before

# Diff Tab
- Removed the jQuery AJAX call and replaced with htmx
- Tested the diff tab functionality and it works as before

## htmx Attributes
- `hx-post="{{.RepoLink}}..."`: make a POST request to the endpoint
- `hx-indicator=".tab[data-tab='diff']"`: attach the loading indicator
to the tab body
- `hx-target=".tab[data-tab='diff']"`: target the tab body for swapping
with the response
- `hx-swap="innerHTML"`: swap the target's inner HTML
- `hx-include="#edit_area"`: include the value of the textarea (content)
in the request body
- `hx-vals='{"context":"{{.BranchLink}}"}'`: include the context in the
request body
- `hx-params="context,content"`: include only these keys in the request
body

# Demo using `fetch` and `htmx` instead of jQuery AJAX

![demo](585cd6e8-f329-4c9e-ab53-a540acbd7988)

---------

Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: silverwind <me@silverwind.io>
(cherry picked from commit c1331d1f7ab60249ed2f080b24f3e32093fa708d)

templates/repo/editor/edit.tmpl

@@ -30,7 +30,7 @@
 					<a class="active item" data-tab="write">{{svg "octicon-code"}} {{if .IsNewFile}}{{ctx.Locale.Tr "repo.editor.new_file"}}{{else}}{{ctx.Locale.Tr "repo.editor.edit_file"}}{{end}}</a>
 					<a class="item" data-tab="preview" data-url="{{.Repository.Link}}/markup" data-context="{{.RepoLink}}/src/{{.BranchNameSubURL}}" data-markup-mode="file">{{svg "octicon-eye"}} {{ctx.Locale.Tr "preview"}}</a>
 					{{if not .IsNewFile}}
-					<a class="item" data-tab="diff" data-url="{{.RepoLink}}/_preview/{{.BranchName | PathEscapeSegments}}/{{.TreePath | PathEscapeSegments}}" data-context="{{.BranchLink}}">{{svg "octicon-diff"}} {{ctx.Locale.Tr "repo.editor.preview_changes"}}</a>
+					<a class="item" data-tab="diff" hx-params="context,content" hx-vals='{"context":"{{.BranchLink}}"}' hx-include="#edit_area" hx-swap="innerHTML" hx-target=".tab[data-tab='diff']" hx-indicator=".tab[data-tab='diff']" hx-post="{{.RepoLink}}/_preview/{{.BranchName | PathEscapeSegments}}/{{.TreePath | PathEscapeSegments}}">{{svg "octicon-diff"}} {{ctx.Locale.Tr "repo.editor.preview_changes"}}</a>
 					{{end}}
 				</div>
 				<div class="ui bottom attached active tab segment" data-tab="write">
@@ -45,7 +45,7 @@
 					{{ctx.Locale.Tr "loading"}}
 				</div>
 				<div class="ui bottom attached tab segment diff edit-diff" data-tab="diff">
-					{{ctx.Locale.Tr "loading"}}
+					<div class="tw-p-16"></div>
 				</div>
 			</div>
 			{{template "repo/editor/commit_form" .}}