* Do not show the "Sign up for free" button in issue tracker

The "Sign in to comment" link is good enough and will correctly
show or not show the "Sign Up" button link for those not having
an account already.

Fixes #3407 (link to nowhere when registration is disabled)

* Move html from translation to template

* Remove extra space in `{{ .SignInLink }}`.

If anonymous (not signed in user) requests page
"/:owner/:repo/pulls" he gets an error:

  template: repo/issue/list:11:105: executing "repo/issue/list" at <.PullRequestCtx.Base...>: nil pointer evaluating *models.Repository.Link

This commit fixes that.

Signed-off-by: Dmitry Yu Okunev <dyokunev@ut.mephi.ru>

- add border-radius to filter list items
- use color as background of label for issue/pull view page

Similar to GitHub:
https://github.com/blog/2111-issue-and-pull-request-templates

Priority:
- root
- .gogs
- .github

This commit improves templates readability, since all of them use consistent
indent with all template command blocks indented too.

1. Indents both HTML containers such as <div>, <p> and Go HTML template blocks
   such as {{if}} {{with}}

2. Cleans all trailing white-space

3. Adds trailing last line-break to each file

Merges repo/sidebar.tmpl with repo/header.tmpl and makes every repo page use
middleware.RepoRef() necessary to display information on this menu.

This is more inline with way GitHub looks like and feels much more natural and
in style with rest of the interface.