[ACA-2064] support custom icons for extensions

[DenysVuika]

PR Checklist

Please check if your PR fulfills the following requirements:

- [x] The commit message follows our guidelines: https://github.com/Alfresco/alfresco-content-app/blob/master/CONTRIBUTING.md#commit
- [ ] Tests for the changes have been added (for bug fixes / features)
- [ ] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application / Infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: ACA-2064, ACA-2036

What is the new behavior?

• allow registering and using custom SVG icons
• update documentation with 'custom icons' section
• update various icons (view, link, move)
• fix issues with 'join/leave library' buttons

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (development@ec3eeb7). Click here to learn what that means.
The diff coverage is 74.28%.

@@              Coverage Diff               @@
##             development     #864   +/-   ##
==============================================
  Coverage               ?   63.57%           
==============================================
  Files                  ?      117           
  Lines                  ?     4052           
  Branches               ?      678           
==============================================
  Hits                   ?     2576           
  Misses                 ?     1352           
  Partials               ?      124

Impacted Files	Coverage Δ
src/app/components/common/common.module.ts	100% <ø> (ø)
src/app/extensions/core.extensions.module.ts	80% <ø> (ø)
...app/components/context-menu/context-menu.module.ts	100% <ø> (ø)
src/app/components/common/icon/icon.component.ts	100% <100%> (ø)
...mponents/layout/app-layout/app-layout.component.ts	65.07% <100%> (ø)
src/app/directives/directives.module.ts	100% <100%> (ø)
...in-library/toggle-join-library-button.component.ts	85.71% <100%> (ø)
src/app/extensions/extension.service.ts	62.08% <55.55%> (ø)
...join-library/toggle-join-library-menu.component.ts	66.66% <66.66%> (ø)

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