Add REST endpoints for bulk tagging & un-tagging of projects

[nscuro]

Description

Adds REST endpoints for bulk tagging & un-tagging of projects.

Addressed Issue

Relates to #586

Additional Details

Bulk operations are currently limited to 100 projects per request. We can raise this limit if users need it. But for a beginning it's better to stay conservative since limiting it later would mean a breaking change.

Both POST and DELETE endpoints are implemented to be atomic operations.

Portfolio ACLs are considered:

• Trying to tag projects the user has no access to is a no-op
• Trying to un-tag projects the user has no access to is a no-op

There will be no HTTP 403 response if tagging or un-tagging of inaccessible projects is attempted.

Checklist

• [x] I have read and understand the contributing guidelines
• ~This PR fixes a defect, and I have provided tests to verify that the fix is effective~
• [x] This PR implements an enhancement, and I have provided tests to verify that it works as intended
• ~This PR introduces changes to the database model, and I have added corresponding update logic~
• ~This PR introduces new or alters existing behavior, and I have updated the documentation accordingly~

[codacy-production[bot]]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
:white_check_mark: +0.08% (target: -1.00%)	:white_check_mark: 92.22% (target: 70.00%)

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (a0407b46afd55bd77d02712ff0ebdab58f0f1991) | 21903 | 16650 | 76.02% | | | Head commit (c41717f515d8eb5bb942fb9bf228d3a46ffc8a58) | 21963 (+60) | 16714 (+64) | 76.10% (**+0.08%**) | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#3894) | 90 | 83 | **92.22%** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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