Turns out we don't really need it - the logic to use
all elements from the group list belongs to higher-level
code like the `sync` or `foreach` implementation

Do not use `gitlab` and `github_enterprise` nested classes, just use
`gitlab_url` and `github_enterprise_url` attributes.

Also, get rid of the named types in tsrc/manifest.py - one of them
(ManifestConfig) had the same name as an other class! If anything, we
should use TypedDict instead

Finally, rename vague `.load()` method into `apply_config()`

LocalManifest class:
* no longer hold any configuration, and only knows how to
update the manifest clone in <workspace>/.tsrc/manifest. All configuration
attributes are now managed by the WorkspaceConfig class
* rename confusing `manifest` property into a `get_manifest()` method

Workspace class:
* contains a LocalManifest and a WorkspaceConfig instance.
* instead of having to call `load_manifest`, the config file in
`<workspace>/.tsrc/config.yml` is parsed in Workspace ctor
* remove some delegation methods that were reading config from
  self.local_manifest and either use
    - self.config or
    - self.local_manifest.get_manifest()

Also, no longer handle local manifest commit from a *path* and not an url, and
get rid of the concept of a manifest *tag* altogether.

Finally, get rid of the confusing "active_groups" naming. It's now in WorkspaceConfig.repo_groups
which is hopefully a better location and nome.

* `tsrc init` can now only be called once, and always with an URL
* Get rid of the `--file` option

Instead of running `init` twice, we need too
check what happens when we change the manifest
branch in the workspace config.

Fix #180, #178

Fix #186

Before:
  tsrc init --group foo --group bar

After:
   tsrc init --groups foo bar

This adds links from the doc to the GitHub repo, and also an "Edit this page
on GitHub" link, which is nice :)