Bug 23975: Add ability to search and install plugins from GitHub

Bug 23975: Add ability to search and install plugins from GitHub

It would be nice to be able to search and install plugins directly from GitHub ( and GitLab if possible ). This would make it easier to verify the authenticity of plugins and make discovering new plugins easier as well as making installation of plugins simpler.

Test Plan:
1) Apply this patch
2) Add the new block for plugin_repos to your koha-conf.xml
3) Restart all the things!
4) Browse to the plugins home page
5) Try searching for 'patrons' in the new search box
6) Verify you get results!

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>