Add a configuration for templates

[jma]

Is your feature request related to a problem? Please describe.

The templates are hard coded, thus it is not possible to override:

invenio-oauth2server/invenio_oauth2server/views/server.py

Line 79 in f3bc295

return render_template("invenio_oauth2server/authorize.html", **ctx)

Describe the solution you'd like

It would be great to have a variable in the config.py file.

Thanks in advance.