JSXC for Nextcloud v4.2.0

Added

• update JSXC to 4.2.0 (changelog)
• use Nextcloud color scheme
• bump max Nextcloud version to 20

Fixed

• #159 do not restore accounts on login page
• add chat login only to main login page
• fix roster space for files table
• fix correct use of ocp/util
• increase timeout to register managed server
• fix hosts with custom port
• fix fast repeating locks
• jsxc/jsxc#916 fix null xml attribute

Misc

• run tests with master and php 7.4
• update ci to bionic
• replace deprecated annotations
• fix integration tests with random values
• add missing imports
• update composer dependencies

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