Add option to refresh user data from GitHub (Fix #339)

Since users might change their avatars (see: https://github.com/blog/1803-switch-your-picture-with-ease) or other data, this will add an option to "refresh" the data stored on jissues with that from GitHub.

**Note** that avatars might get cached from your browser, so be sure to <kbd>F5</kbd> or <kbd>Ctrl</kbd>+<kbd>R</kbd> after the refresh is done :wink: