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-status.service.js

@@ -21,6 +21,7 @@ module.exports = class GithubCommitStatus extends LegacyService {
   static get route() {
     return {
       base: 'github/commit-status',
+      pattern: ':user/:repo/:branch/:commit',
     }
   }
 
@@ -28,7 +29,6 @@ module.exports = class GithubCommitStatus extends LegacyService {
     return [
       {
         title: 'GitHub commit merge status',
-        pattern: ':user/:repo/:branch/:commit',
         namedParams: {
           user: 'badges',
           repo: 'shields',