Improve the people invite screens

[giomfo]

see element-hq/element-ios#904 for details

[giomfo]

Like in ios Riot, we should add a + icon on the right side of the "invitable" contact cells.
See

[giomfo]

Note: When a contact is removed from the "Known Contacts" section because a local contact matches with his Matrix identifier. The local contact should display the matrix avatar of this user.

[giomfo]

The provided tab ("Matrix contacts" | "All contacts") is not adapted to the global search screen. We suggest adding a specific tick in the Local contacts header, see the following screenshot:

Note: The local contacts section header must be renamed "Local Contacts" in android Riot