[stackexchange] Make author anonymization optional for stackexchange

[jjmerchante]

This code makes it optional to pseudo-anonymize the owner of question and answers. It creates a hash of the name and user_id and remove author_link and profile_image.

For enabling the anonymization, include the following parameter in the stackexchange section:

[stackexchange]
...
anonymize = true

[coveralls]

Pull Request Test Coverage Report for Build 635280104

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 82.224%

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Files with Coverage Reduction	New Missed Lines	%
/home/runner/work/grimoirelab-elk/grimoirelab-elk/grimoire_elk/enriched/github.py	2	75.0%
<!--	Total:	2		-->

Totals
Change from base Build 635269469:	0.02%
Covered Lines:	8608
Relevant Lines:	10469

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💛 - Coveralls