feat: add support for /import/github

On startup, the `timeout` parameter is loaded from config and stored on
the base gitlab object instance. This instance parameter is used as the
timeout for all API requests (it's passed into the `session` object when
making HTTP calls).

This change allows any API method to specify a `timeout` argument to
`**kwargs` that will override the global timeout value. This was
somewhat needed / helpful for the `import_github` method.

I have also updated the docs accordingly.

Addresses python-gitlab/python-gitlab#952

This adds a method to the `ProjectManager` called `import_github`, which
maps to the `/import/github` API endpoint. Calling `import_github` will
trigger an import operation from <repo_id> into <target_namespace>,
using <personal_access_token> to authenticate against github. In
practice a gitlab server may take many 10's of seconds to respond to
this API call, so we also take the liberty of increasing the default
timeout (only for this method invocation).

Unfortunately since `import` is a protected keyword in python, I was unable
to follow the endpoint structure with the manager namespace. I'm open to
suggestions on a more sensible interface.

I'm successfully using this addition to batch-import hundreds of github
repositories into gitlab.

Nicer stacktrace

feat: add more options for project/group ci variables manipulation

docs(projects): fix file deletion docs

The function `file.delete()` requires `branch` argument in addition to `commit_message`.

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