Title for link custom field to project section

Update generated HTML in Custom Field edit page to display a title on
top of the section to manage links to projects.

Minor changes to language strings (adjust the labels):
- linked_projects_label: remove extra, unneeded ':'
- link_custom_field_to_project_title: plural form for 'project'

Fixes #20096

lang/strings_english.txt

@@ -1287,9 +1287,9 @@ $s_custom_field_require_report = 'Required On Report';
 $s_custom_field_require_update = 'Required On Update';
 $s_custom_field_require_resolved = 'Required On Resolve';
 $s_custom_field_require_closed = 'Required On Close';
-$s_link_custom_field_to_project_title = 'Link custom field to project';
+$s_link_custom_field_to_project_title = 'Link custom field to projects';
 $s_link_custom_field_to_project_button = 'Link Custom Field';
-$s_linked_projects_label = 'Linked Projects:';
+$s_linked_projects_label = 'Linked Projects';
 $s_custom_field_sequence = 'Sequence';
 $s_custom_field_sequence_label = 'Sequence';
 $s_custom_field_type_enum_string = '0:String,1:Numeric,2:Float,3:Enumeration,4:E-mail,5:Checkbox,6:List,7:Multiselection list,8:Date,9:Radio,10:Textarea';

manage_custom_field_edit_page.php

@@ -212,16 +212,21 @@
            * We may need to add extra checks to exclude projects from the list that the user
 		   * can't link/unlink fields from/to. */
 ?>
-<div class="form-container">
-	<div id="custom-field-link-project" class="field-container">
-		<span class="display-label"><span><?php echo lang_get( 'link_custom_field_to_project_title' ) ?></span></span>
-		<div class="display-value">
-			<?php print_custom_field_projects_list( $f_field_id ) ?>
-		</div>
-		<span class="label-style"></span>
-	</div>
-	<form method="post" action="manage_custom_field_proj_add.php">
+<div id="manage-custom-field-link-div" class="form-container">
+	<form id="manage-custom-field-link-form" method="post" action="manage_custom_field_proj_add.php">
 		<fieldset>
+			<legend><span><?php echo lang_get( 'link_custom_field_to_project_title' ) ?></span></legend>
+
+			<div id="custom-field-link-project" class="field-container">
+				<span class="display-label">
+					<span><?php echo lang_get( 'linked_projects_label' ) ?></span>
+				</span>
+				<div class="display-value">
+					<?php print_custom_field_projects_list( $f_field_id ) ?>
+				</div>
+				<span class="label-style"></span>
+			</div>
+
 			<input type="hidden" name="field_id" value="<?php echo $f_field_id ?>" />
 			<?php echo form_security_field( 'manage_custom_field_proj_add' ); ?>
 			<div class="field-container">