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add bootstrap frontend

See merge request !2

- Added explanation about service in index
- Split into "login_providers" and "verification_providers"
- Added stub "issue_credential" page
- Unified verifier and user profile session data
	- TODO: refactor verifier struct into enum (i.e. turn it inside
out)
	- maybe need to do struct containing metadata and enum as fields

This reverts commit 5208fb9f.

This reverts commit c9a600cf.

- Move Oauth flow into
	- `make_X_client` - configure the `oauth2::BasicClient`
	- `redirect_X_auth_view` - redirect user to oauth service server
	- `user_authorized_x_view` - acquire oauth token from oauth service callback, store in session, redirect user
		- This is the view whose url should correspond to the
one set in the service's authorized redirect urls.

These can probably be generalized into a factory of some sort, like a
flask blueprint. The design is very similar to the flask-dance library.

Actix oauth server

See merge request !1