Consolidate dropdown usage

[cloverich]

Between #124 and the existing toolbar dropdowns and context menu in #209 there are multiple implementations / variations of the dropdown. The journal selector dropdown uses the (otherwise nice) evergreen-ui one. Once the features are all implemented, refactor dropdowns to all use the same base / styles as appropriate.

List of where dropdowns exist:

• Toolbar
• Context menu in sidebar
• Journal selector
• Editor UI (for Internal note linking #124 )

Also after #223 -- I need to kick out or find a long-term home for ariakit combobox (used for the note linking dropdown, b/c of the usual time constraints)

This may also be needed for the #217 because it uses a dropdown for typeahead.