Fix milestone assignment in New Issue

Avoids the use of HTMX on milestone assignment within a New Issue form.

The New Issue form doesn't have an issue ID to send to a milestone change URL,
which the backend expects in order to construct a proper reply. The frontend
template was also not built to use the new HTMX response. This resulted in a
backend error and a large warning whenever anyone tried to set a milestone
from within the New Issue form (new pull requests were also affected), rather
than from a View Issue page.

This introduces a new parameter into the `issue/milestone/select_menu`
template, "NewIssuePage".

When unset, the template produces the same results as before. Selection uses
`hx-post` to notify the server immediately, using the updated htmx fragment
from the reply.

When set to a truthy value, the old style of form is used. Selection uses
`data-id` and `data-href` to update the selected milestone locally, via
`selectItem` in `repo-legacy.js`, recreating the selected element and updating
the hidden form value.

Fixes #5176.

templates/repo/issue/milestone/select_menu.tmpl

@@ -1,3 +1,4 @@
+{{$useHTMX := not .NewIssuePage}}
 {{if or .OpenMilestones .ClosedMilestones}}
 	<div class="ui icon search input">
 		<i class="icon">{{svg "octicon-search" 16}}</i>
@@ -5,7 +6,7 @@
 	</div>
 	<div class="divider"></div>
 {{end}}
-<div class="no-select item" hx-post="{{$.RepoLink}}/issues/milestone?issue_ids={{$.Issue.ID}}&htmx=true">{{ctx.Locale.Tr "repo.issues.new.clear_milestone"}}</div>
+<div class="no-select item"{{if $useHTMX}} hx-post="{{$.RepoLink}}/issues/milestone?issue_ids={{$.Issue.ID}}&htmx=true"{{end}}>{{ctx.Locale.Tr "repo.issues.new.clear_milestone"}}</div>
 {{if and (not .OpenMilestones) (not .ClosedMilestones)}}
 	<div class="disabled item">
 		{{ctx.Locale.Tr "repo.issues.new.no_items"}}
@@ -17,7 +18,7 @@
 			{{ctx.Locale.Tr "repo.issues.new.open_milestone"}}
 		</div>
 		{{range .OpenMilestones}}
-			<a class="item" hx-post="{{$.RepoLink}}/issues/milestone?id={{.ID}}&issue_ids={{$.Issue.ID}}&htmx=true">
+			<a class="item"{{if $useHTMX}} hx-post="{{$.RepoLink}}/issues/milestone?id={{.ID}}&issue_ids={{$.Issue.ID}}&htmx=true"{{else}} data-id="{{.ID}}" data-href="{{$.RepoLink}}/issues?milestone={{.ID}}"{{end}}>
 				{{svg "octicon-milestone" 16 "tw-mr-1"}}
 				{{.Name}}
 			</a>
@@ -29,7 +30,7 @@
 			{{ctx.Locale.Tr "repo.issues.new.closed_milestone"}}
 		</div>
 		{{range .ClosedMilestones}}
-			<a class="item" hx-post="{{$.RepoLink}}/issues/milestone?id={{.ID}}&issue_ids={{$.Issue.ID}}&htmx=true">
+			<a class="item"{{if $useHTMX}} hx-post="{{$.RepoLink}}/issues/milestone?id={{.ID}}&issue_ids={{$.Issue.ID}}&htmx=true"{{else}} data-id="{{.ID}}" data-href="{{$.RepoLink}}/issues?milestone={{.ID}}"{{end}}>
 				{{svg "octicon-milestone" 16 "tw-mr-1"}}
 				{{.Name}}
 			</a>

templates/repo/issue/new_form.tmpl

@@ -64,7 +64,7 @@
 				{{end}}
 			</span>
 			<div class="menu">
-				{{template "repo/issue/milestone/select_menu" .}}
+				{{template "repo/issue/milestone/select_menu" dict "." . "NewIssuePage" 1}}
 			</div>
 		</div>
 		<div class="ui select-milestone list">