Replace remaining fontawesome dropdown icons with SVG (#24455)

- Replace leftover dropdown triangles with SVG
- Replace remove icon with SVG and add styling for it:

<img width="817" alt="Screenshot 2023-05-01 at 00 40 05"
src="https://user-images.githubusercontent.com/115237/235379271-4674d4f7-b11e-4d6d-90f9-1478325443ca.png">
<img width="816" alt="Screenshot 2023-05-01 at 00 46 56"
src="https://user-images.githubusercontent.com/115237/235379451-b515afb3-9773-4f6f-a259-e7048235bcba.png">

templates/repo/graph.tmpl

@@ -8,7 +8,6 @@
 				<div class="ui icon buttons tiny color-buttons">
 					<div class="ui multiple selection search dropdown" id="flow-select-refs-dropdown">
 						<input type="hidden" name="flow">
-						<i class="dropdown icon"></i>
 						<div class="default text">{{.locale.Tr "repo.commit_graph.select"}}</div>
 						<div class="menu">
 							<div class="item" data-value="...flow-hide-pr-refs">

templates/repo/issue/fields/dropdown.tmpl

@@ -3,7 +3,8 @@
 	{{/* FIXME: required validation */}}
 	<div class="ui fluid selection dropdown {{if .item.Attributes.multiple}}multiple clearable{{end}}">
 		<input type="hidden" name="form-field-{{.item.ID}}" value="0">
-		<i class="dropdown icon"></i>
+		{{svg "octicon-triangle-down" 14 "dropdown icon"}}
+		{{svg "octicon-x" 14 "remove icon"}}
 		<div class="default text"></div>
 		<div class="menu">
 			{{range $i, $opt := .item.Attributes.options}}