parser: Parse "Depends-on" tags in emails

parser: Parse "Depends-on" tags in emails

Add a new function to parse "Depends-on" tags to the parser. The value
may either be the message ID of a patch or cover letter email already
received by Patchwork, or the web URL of a patch or series. When this
tag is found, the parser will add the series (or the series the patch
belongs to) as a dependency to the series it is creating.

This parser feature is only active when the feature flag is enabled on
the project related to the patch or cover letter.

Signed-off-by: Adam Hassick <ahassick@iol.unh.edu>
Acked-by: Aaron Conole <aconole@redhat.com>
[stephenfin: Parse patch dependencies unconditionally]
Signed-off-by: Stephen Finucane <stephen@that.guru>