List available frontends also when no static/frontends folder is present yet

Fixes admin-fe#201 (closed)

• To see what front ends are installed, it does ls static/frontends. When this folder doesn't exists yet, it will now return an empty array.
• Installing still works since the folder is created during installation already

added 1 commit

• 11925446 - fix after running `mix credo --strict --only=warnings,todo,fixme,consistency,readability`

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changed milestone to %2.4.1

added fix: to be merged in stable label

Otherwise looks good to me.

resolved all threads

added 1 commit

• 63e4dddd - After code review

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mentioned in commit 84ec0fbe

merged

mentioned in commit 53b0dd4e