Rename server directory to keyserver

Summary: This diff represents set of files that location changed after renaming server to keyserver but didn't need additional changes link imports, error messages etc. CI should fail for this differential but we accept it since the following diff in the stack will fix it by providing changes in imports and error messages in files that need it. This decision weas made for reviewer convenience.

Test Plan: There is no test plan for this diff. We can assume it is correct if the following diff in the stack passes CI and makes android, web and iOS buildable.

Reviewers: palys-swm, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: benschac, ashoat, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3314