Only ensure a CbrainFileList read access at task launch, resolves #1057

[MontrealSergiy]

see #1057 for details

[prioux]

Looks good to me.

[MontrealSergiy]

Hi, thanks. Indeed technically feature seems working, and task launch renders task submission forms successfully. Still there could be more problems down the road: task submission, validation, etc whenever file access is double-checked. (Repeat check servers either as anti-hack or access change support I guess). It may deserve a separate ticket, but if you like I can add they here. I think rest of task access check should be modified, case by case ( or, if you prefer keep code dried, by adding skip_cbcvs / skip_lists etc param to find_accessible_userfiles method, to keep it drier ).

[MontrealSergiy]

I am a bit unsure with files that have cbcsv extension yet not type

[MontrealSergiy]

@prioux I pushed a few changes to make sure that the task with read-only list can pass the validation