Added caching of Github credentials

[chownces]

Description

DO NOT MERGE YET (potential security risk)

I'm not sure how our GitHub OAuth app has been configured, but by default the access tokens do not have an expiry date... The optional setting for expiring tokens as stated in GitHub's documentation seems to be missing too.

This is potentially very dangerous, as we are storing this account-wide access token in local storage.

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How to test

Log in to our GitHub Assessments page and refresh the browser.

Checklist

• I have tested this code
• I have updated the documentation

[coveralls]

Pull Request Test Coverage Report for Build 967129830

• 8 of 9 (88.89%) changed or added relevant lines in 4 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 30.102%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/commons/sagas/GitHubPersistenceSaga.ts	1	2	50.0%

Files with Coverage Reduction	New Missed Lines	%
src/commons/sagas/GitHubPersistenceSaga.ts	1	13.51%

Totals
Change from base Build 966753988:	0.03%
Covered Lines:	3213
Relevant Lines:	9833

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1035823582

• 8 of 9 (88.89%) changed or added relevant lines in 4 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 30.041%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/commons/sagas/GitHubPersistenceSaga.ts	1	2	50.0%

Files with Coverage Reduction	New Missed Lines	%
src/commons/sagas/GitHubPersistenceSaga.ts	1	13.51%

Totals
Change from base Build 1028531753:	0.03%
Covered Lines:	3201
Relevant Lines:	9811

---

💛 - Coveralls

[martin-henz]

security concerns discussed (and dismissed) in telegram "SA 2122 Summer Development"