Use a user/group suggestion component for participants

Create a new component to select users and/or groups as participants.

The component should behave like the "add to collections" one, mixed with "Add guest" from google calendar.

Replace the participants' input with an "Add participants" text and when clicking on it show a search + list, like for collections selection.

We should be able to extract a generic component to be used for users, groups, directory entries, documents or collections selection...

On task screen :

• When tapping the « participants » field, the add participant screen is displayed and the keyboard is deployed.

On add participant screen :

• When no participants are added, the « ok » button is disabled and the search field Icon is a magnifier, and the rest of the screen is empty.
• When participants are already added to the task, a title « Added participants » is visible, as well as the list of participants with a « user » avatar icon for users, « group » avatar icon for groups and a cross (delete).
• When tapping a participant "cross", the corresponding participant is deleted from the list
• When typing in the search field, the search results are displayed as a list with user names and corresponding avatars (user/group icons), and the search field icon becomes a clickable cross.
• When the find list is empty, a "no groups or users found" text is displayed
• When clicking a search result, the search field is emptied and the corresponding participant is added to the list
• On tapping "done" (but not back), the task screen is displayed, the participant field is visible, and the list of participants is updated
• On tapping "back" button, actions are cancelled (cf. meeting 21st of November)