Minor UI tweaks

[Larivact]

• Gitea version (or commit ref): 1.7.0

Behavior

• templates/repo/create.tmpl – Once you select a license you can no longer deselect it.
• External issue trackers: The issues tab should have an external-link-alt icon and link directly to the external tracker so that you can see the URL by hovering over it.
• When you select an issue/pull request the toggle button transitions to open & close buttons, which is not only problematic because it still looks like a toggle button but also doesn't make sense because you are never able to open or close an issue/pull request, it's either open or closed, you only ever have one choice (to toggle the current state). I suggest the transition to be removed and instead display an open or close button next to the toggle button.
  
  
• The above check boxes should not be displayed for repositories where you are not allowed to open or close issues/pull requests.
• 404 pages of files, issues, pull requests, releases and wiki pages should include the repository header if the repository exists.
• Reset password should directly tell you when a code is invalid: https://try.gitea.io/user/reset_password?code=x
• public/js/index.js – When editing the labels of an existing issue (see screenshot), the labels are currently saved with every select / deselect, which just leads to unnecessary spam. The labels should only be saved after closing the popup, which is also the way GitHub does it.
  

Visuals

• templates/repo/header.tmpl – Visually disable fork button on own repositories (you cannot clone them) by adding disabled class .
  
  
• templates/repo/home.tmpl – Give New Pull Request button a text label instead of a cryptic icon.
  
• public/less/themes/arc-green.less – The dark theme should use a less vibrant color for green buttons, I suggest #87ab63, the same color used for links.

[jolheiser]

I think I'll give this a go. I'll make a WIP PR once I've made some progress.

EDIT: The PR can be followed at #5980