Correctly generate link for editing user

With PMA_URL_getCommon we get correct separator and escaping for free.
Correctly use PMA_URL_getCommon instead of manually adding some parameters
Correctly generate link for revoking user

With PMA_URL_getCommon we get correct separator and escaping
for free.
Correctly generate link for exporting user

With PMA_URL_getCommon we get correct separator and escaping
for free.
Documentation and coding style improvements
Various coding style fixes
Prevent manual tweaking of url parameters

PMA_URL_getCommon provides better inteface to constuct them