Improve error messages on admin-only pages for non-admins

[vdavid]

Our admin-only pages currently don't handle non-admin visits nicely.

See these three:

Subscription manage – /cody/subscription/manage:

Team manage – /cody/team/manage:

Add seats – /cody/manage/subscription/new?addSeats=1:

We need designs for all of these states and a front-end implementation.

[chenkc805]

@david.veszelovszki can you provide a short description for each of these error states:

• Why these errors come up?
• What needs to be done (either by the user or us) to fix?

[chenkc805]

@vdavid can you answer these questions? ^^