Send guestlink emails

[aduffeck]

We now send the guest links to the according recipient using the notification service.

This PR also enables the guestlink capability when the feature is enabled (requires opencloud-eu/reva#857).

Fixes #3616
Fixes #3629

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

View in Codacy

🟢 Metrics 10 complexity

Metric	Results
Complexity	10

View in Codacy

🟢 Coverage 50.75% diff coverage · +0.03% coverage variation

Metric	Results
Coverage variation	✅ +0.03% coverage variation (-1.00%)
Diff coverage	✅ 50.75% diff coverage

View coverage diff in Codacy

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (3a55525)	91096	22381	24.57%
Head commit (3858d7e)	91163 (+67)	22425 (+44)	24.60% (+0.03%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#3668)	67	34	50.75%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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