refactor EnterpriseServerApi for single-fetching of fesUrl

[tomholub]

I think that after finishing and merging #1331 we should also update EnterpriseServerApi to have email property (which will be passed from AppContextWithUser), so we don't need to pass it manually for every method.
Then we'll be able to fetch fesUrl only once and store result locally, while currently it's fetched for each request.

Originally posted by @sosnovsky in #1342 (comment)