VE-541 Modify how Parsoid handles the API calls

## Overview
The scope of this PR includes modifying `ParserService.js` and `mediawiki.parser.environment.js` to accept URIs for api configs in addition to the current functionality of being able to pass a canonical `prefix`. To accomplish this, modifications have been made in:
* [`MWParserEnvironment.getParserEnv`](https://github.com/Wikia/mediawiki-extensions-Parsoid/compare/master...ve-541-parsoid-custom-config?expand=1#diff-3a649c2384d417c3c5ae4ebad7ee6404L266)
* [`MWParserEnvironment.prototype.switchToConfig`](https://github.com/Wikia/mediawiki-extensions-Parsoid/compare/master...ve-541-parsoid-custom-config?expand=1#diff-3a649c2384d417c3c5ae4ebad7ee6404L314)

In addition, a simple helper method to match URIs was added to `MWParserEnvironment`: [`MWParserEnvironment.prototype.isApiSourceAUri`](https://github.com/Wikia/mediawiki-extensions-Parsoid/compare/master...ve-541-parsoid-custom-config?expand=1#diff-3a649c2384d417c3c5ae4ebad7ee6404L305)

The regex for affected routes and variables previously named `prefix` have been generalized to reflect to wider range of accepted inputs.

### Todo
- [x] Update regex for routes
- [ ] Update tests (if applicable)

@inez @kflorence @BladeBronson @lizlux