Misc Backend Cleanup 2

[rc-davis]

The following miscellaneous cleanup items were identified when working on Tooling issue 453.

This continues where #461 leaves off.

• re-export middleware from middleware/index.ts.
• Move authenticatedSites into a new subdir authenticated/sites and merge authenticated/sites.js into the new authenticated/sites/index.js. (also setup subrouters in server.js?)
• Distribute identity services into other sub-directories
• Rename one either services/identity/SitesService or services/utilServices/SitesService
• Rename MailClient to MailService (same for SmsClient?)
• Stop mixing import and require in server.js
• Create a FormsgService that contains most of FormsgRouter.formsgCreateSite()
• Remove apiTokenName from Sites table.
• Add unit and integration tests for newly introduced services and routes

[seaerchin]

we should also add unit + integration tests for the newly introduced services/routes in #456 and #459