Reduce configuration items

I think these items are not necessary.

• chatDisabled
• showInstanceSpecificPanel
• scopeOptionsEnabled
• formattingOptionsEnabled
• collapseMessageWithSubject

Because...

• Try to connect the socket chat:public. If it succeeds, show the chat panel.
• Try to retrieve /instance/panel.html. If it succeeds, show the instance specific panel.
• Scope and formatting options are enabled for Pleroma BE, disabled for GNU social FE. Retrieve /nodeinfo/2.0.json and check software/name equals to pleroma.
• collapseMessageWithSubject is configurable for users. Instance-wise configuration is redundant.

See also !330 (closed)