Add support for GitLab and Bitbucket

[drpatelh]

Description of feature

A number of users would like to use other Git providers in conjunction with nf-core/tools. However, given the origins of nf-core the tooling is quite heavily tied in with Github and the Github API. It would be great to make this more flexibile to work with all of the tools commands.

A good start will be to create nf-core organisation repos on both of those provider sites so we can push some pipelines and set-up automated tests in the future.

@ewels had some ideas about how to make this happen so I will let him fill out this issue with an essay.

[ewels]

The majority of interaction with the GitHub API is done with the nf-core modules commands. So the suggestion was to start there and refactor how that code works with the central modules repository.

Instead of using the API to fetch file contents and metadata on the fly, it could clone any arbitrary git repository (from GitLab or wherever) and then work with local files.

The main downside of this approach is that every operation will require a git pull to begin with, to ensure that the local copy is up to date. However, after the initial clone this should hopefully be relatively quick and we can probably show a progress bar.

The upside of doing this is better compatibility with git providers and also less dependency on the API being up and being within rate limits.

Once this is done we can investigate where else the API is used.