Refactor some Str2html code (#29397)

This PR touches the most interesting part of the "template refactoring".

1. Unclear variable type. Especially for "web/feed/convert.go":
sometimes it uses text, sometimes it uses HTML.
2. Assign text content to "RenderedContent" field, for example: `
project.RenderedContent = project.Description` in web/org/projects.go
3. Assign rendered content to text field, for example: `r.Note =
rendered content` in web/repo/release.go
4. (possible) Incorrectly calling `{{Str2html
.PackageDescriptor.Metadata.ReleaseNotes}}` in
package/content/nuget.tmpl, I guess the name Str2html misleads
developers to use it to "render string to html", but it only sanitizes.
if ReleaseNotes really contains HTML, then this is not a problem.

templates/user/dashboard/milestones.tmpl

@@ -141,7 +141,7 @@
 							</div>
 							{{if .Content}}
 								<div class="markup content">
-									{{.RenderedContent|Str2html}}
+									{{.RenderedContent}}
 								</div>
 							{{end}}
 						</li>

templates/user/profile.tmpl

@@ -29,7 +29,7 @@
 				{{else if eq .TabName "followers"}}
 					{{template "repo/user_cards" .}}
 				{{else if eq .TabName "overview"}}
-					<div id="readme_profile" class="markup">{{.ProfileReadme | Str2html}}</div>
+					<div id="readme_profile" class="markup">{{.ProfileReadme}}</div>
 				{{else}}
 					{{template "explore/repo_search" .}}
 					{{template "explore/repo_list" .}}