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 you browser, so be sure to <kbd>F5</kbd> or <kbd>Ctrl</kbd>+<kbd>R</kbd> :wink: