When adding/editing a project if the backend if Github, adjust the label and show version_url
Reset the label to its default value
Define call_url and a method call_url to BaseBackend allowing us to set a consistent User-Agent for all our requests
Update the plugins that are making requests to use call_url
Move the default regex at the top of the backends module
Adjust all the backends using the default regex to retrieve it from the backends module
Project wide pep8 clean up
The backends are not instantiated so we need call_url to be a @classmethod

Simply the call_url by just having it on BaseBackend and let
get_versions_by_regex rely on it
Drop un-used import from the backends
Do not override the previously defined REGEX variable
Remove un-used variable and change from self to cls since the object is not instantiated
Drop the import of requests to rely on call_url and adjust self for cls

The difference between self and cls is that the object is not instantiated
Drop un-used imports and replace self by cls as the object is not instantiated
Drop un-used import
Split get_versions_by_regex into two parts

get_versions_by_regex retrieves the versions from a url

get_versions_by_regex_for_text retrieves the versions from a text
When we have several regex only request the page once to run both regexes
get_versions_by_regex_for_text needs the url for the error messages
Adjust the call to get_versions_by_regex_for_text in the folder and gnu backends