Fix URLJoin, markup render link reoslving, sign-in/up/linkaccount page common data (#36861)

The logic of "URLJoin" is unclear and it is often abused.

Also:
* Correct the `resolveLinkRelative` behavior
* Fix missing "PathEscape" in `ToTag`
* Fix more FIXMEs, and add new FIXMEs for newly found problems
* Refactor "auth page common template data"

templates/package/shared/list.tmpl

@@ -44,7 +44,7 @@
 				{{svg "octicon-package" 48}}
 				<h2>{{ctx.Locale.Tr "packages.empty"}}</h2>
 				{{if and .Repository .CanWritePackages}}
-					{{$packagesUrl := URLJoin .Owner.HomeLink "-" "packages"}}
+					{{$packagesUrl := print .Owner.HomeLink "/-/packages"}}
 					<p>{{ctx.Locale.Tr "packages.empty.repo" $packagesUrl}}</p>
 				{{end}}
 				<p>{{ctx.Locale.Tr "packages.empty.documentation" "https://docs.gitea.com/usage/packages/overview/"}}</p>

templates/repo/blame.tmpl

@@ -1,5 +1,5 @@
 {{if or .UsesIgnoreRevs .FaultyIgnoreRevsFile}}
-	{{$revsFileLink := URLJoin .RepoLink "src" .RefTypeNameSubURL "/.git-blame-ignore-revs"}}
+	{{$revsFileLink := print .RepoLink "/src/" .RefTypeNameSubURL "/.git-blame-ignore-revs"}}
 	{{if .UsesIgnoreRevs}}
 		<div class="ui info message">
 			<p>{{ctx.Locale.Tr "repo.blame.ignore_revs" $revsFileLink "?bypass-blame-ignore=true"}}</p>

templates/user/auth/captcha.tmpl

@@ -10,7 +10,7 @@
 	<div class="inline field tw-text-center required">
 		<div id="captcha" data-captcha-type="g-recaptcha" class="g-recaptcha-style" data-sitekey="{{.RecaptchaSitekey}}"></div>
 	</div>
-	<script defer src='{{URLJoin .RecaptchaURL "api.js"}}'></script>
+	<script defer src='{{.RecaptchaAPIScriptURL}}'></script>
 {{else if eq .CaptchaType "hcaptcha"}}
 	<div class="inline field tw-text-center required">
 		<div id="captcha" data-captcha-type="h-captcha" class="h-captcha-style" data-sitekey="{{.HcaptchaSitekey}}"></div>