Add message on home page with no activity

[mtancoigne]

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Warnings

• This PR will change existing database contents.
• This PR introduces a breaking change to existing installations.

Summary

Display a message when there are no activities to display on the home page

Linked issues

Resolves #2743

Description of changes

• Moves conditions from the activity/* partials to Timeline.local method,
• Display a message on home page when feed list is empty.

Translations for fr/en are included

[mtancoigne]

I'll fix the linting issue in the ERB (I forgot to run it locally 🙃 )