Make publishing a new version based on Github tag even easier

Thanks to poltawski's Github repository plugin, uploading a ZIP package with the plugin directly from Github is already possible. It was discussed several times that the process could be even easier. We are going to implement a new interface that checks for git tags existing in the plugin's github repository (if the maintainer uses it) and allows the maintainer to publish a new version in the Plugins directory based on the tag. This new UI would appear at the same page where the new version is to be uploaded.

• We want to promote the culture of releasing by tagging. We hope this would help to keep consistency between what is uploaded in the plugins directory and what is tagged in the git repository.
• We do not want to publish anything on behalf of the maintainer. The maintainer will still need to log in to the PD and explicitly release a new version there.
• At the moment, the Github v3 API is pretty limited as it does not provide much information about unannotated tags. It seems we can't easily detect "recently added tags" unfortunately. For the start, we will use all tags returned by the API (we can eventually filter them later based on the information about exiting versions based on a tag).