Refactor general server_ui code out of origin_ui

[turetske]

Due to implementing origin_ui first, there is a lot of origin specific code that actually code be more general. This should be refactored into server_ui instead of origin_ui so it can be used by other servers such as the cache, director, or namespace. Some of the code in cmd/origin_serve is also more general than just origin code.

[haoming29]

In #291 I refactored much of the authorization check code in web_ui which can be helpful in implementing this issue

[haoming29]

This can be very helpful for #264 as it will greatly reduce repetitive logic we would create.

[haoming29]

Looks like #355 will be heavily dependent on this PR in order not to generate duplicate code and #355 was marked as part of 7.3.0. @bbockelm Do you think we should prioritize this issue to get #355 unblocked?