Refactor route functions in BaseService (#2860)

The route helper functions are fairly well isolated from the rest of BaseService, with a few convenient entry points. They are easier to test in isolation.

The way the code was written before, `pathToRegexp` was invoked once for every request, which seems inefficient.

`route` was validated when it was used, though it seems more helpful to validate it up front.

This breaks out `_makeFullUrl`, `_regex`, `_regexFromPath` into new helper functions `makeFullUrl`, `assertValidRoute`, `prepareRoute`, and `namedParamsForMatch`.

It adds validation to route, and updates the services without patterns to include one, in order to pass the new validation rules.

services/github/github-commit-activity.service.js

@@ -23,6 +23,7 @@ module.exports = class GithubCommitActivity extends LegacyService {
   static get route() {
     return {
       base: 'github/commit-activity',
+      pattern: ':interval(y|4w|w)/:user/:repo',
     }
   }