Fix/958 better file name handling

[inkhey]

related to tracim/tracim#958

• remove get_label_as_file() which is a duplicate of file_name.
• use new default file extension for content type like '.document.html' + migration script
• add file extension in all endpoints
• repair filename check sql query.

Todo :

• Decide final file extension for both html page and thread.
• renaming on the fly, will be done in another pr: see Renaming new content on the fly instead of disallow content creation when filename check failed tracim#1022

[coveralls]

Pull Request Test Coverage Report for Build 961

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 141 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.0006%) to 91.742%

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Files with Coverage Reduction	New Missed Lines	%
tracim_backend/tests/functional/test_contents.py	3	99.86%
tracim_backend/models/context_models.py	3	99.01%
tracim_backend/app_models/contents.py	3	96.52%
tracim_backend/lib/core/content.py	55	84.01%
tracim_backend/models/data.py	77	82.98%

Totals
Change from base Build 959:	-0.0006%
Covered Lines:	16365
Relevant Lines:	17838

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💛 - Coveralls