[ACA-20] Sidenav - navigate to first child route when expanded for the first time

[pionnegru]

PR Checklist

Please check if your PR fulfills the following requirements:

- [ ] 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?

[ ] Bugfix
[ ] 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: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[codecov-io]

Codecov Report

Merging #798 into development will increase coverage by 0.17%.
The diff coverage is 90.47%.

@@               Coverage Diff               @@
##           development     #798      +/-   ##
===============================================
+ Coverage        60.25%   60.43%   +0.17%     
===============================================
  Files              109      110       +1     
  Lines             3646     3667      +21     
  Branches           602      603       +1     
===============================================
+ Hits              2197     2216      +19     
- Misses            1349     1351       +2     
  Partials           100      100

Impacted Files	Coverage Δ
...pp/components/sidenav/expansion-panel.directive.ts	90.47% <90.47%> (ø)

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