[WIP] convert groups module to routes

DO NOT MERGE.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | #1788
| License       | MIT
| Doc PR        | -

refs #1788
@cmfcmf or @Guite does `"letter" = "[a-zA-Z]|\*"` look right to you? I couldn't figure out a way to restrict the string length to once character though.