* Moved attachaments POST url from /issues/attachments to /attachments

* Implemented attachment upload on release page

* Implemented downloading attachments on the release page

* Added zip and gzip files to default allowed attachments

* Implemented uploading attachments on edit release

* Renamed UploadIssueAttachment to UploadAttachment

Closes #162

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.