add new channel page tests

[jkrusty]

fix naming old test 'check if user can pin message' -> 'check if user can reply message'
add new tests:
'check if user can pin message'
'check if user can unpin message on pinned messages page'
'check if user can unpin message messages page'

added some necessary methods to models

_{Huly®: UBERF-7898}