Remove "Replace selection" checkbox

[dlqqq]

Problem

Same as #864.

Current "replace selection" checkbox implementation is likely broken by #859.

Proposed Solution

• Remove the "replace selection" checkbox
• Allow for text selection to be replaced if a text selection is active in the code action toolbar (which currently only allows for the active cell to be replaced).
• Implement a message-global "Replace selection" action within a hamburger menu at the top of each message (e.g. next to the timestamp). This should observe the same behavior as the Replace button in the code action toolbar, i.e. replace text if a text selection is active, replace cell if there exists an active cell, and render as disabled if neither condition is satisfied.