[WIP] add "through" page 

...so that routes have a chance to reload before loading a page with menus on it. refs #1961 

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | #1961
| License       | MIT
| Doc PR        | -

ping @cmfcmf 
I don't think the flash message is necessary if none of us can think of an easy way to handle it.