[SQL-258][SQL-259] Reaction UI bugfixes and enhancements

[kelvinqian00]

Fix the following bugs in the reaction UI:

• DOM nesting error due to nested <dl> elements
• Condition values are sometimes erased when completing statement path selection
• Statement path selector does not automatically offer new path segment after first changing the condition name
• Cannot add array indexes without path input moving on to the next segment; this is fixed with new numeric input
• Extensions paths are always marked as invalid despite actually being valid, preventing users from saving reactions with extension paths

In addition, perform the following enhancements:

• New conditions are now added to the bottom instead of the top of the list
• Typing used to be blocked when trying to input a duplicate condition name; this is now replaced with an error warning
• Add additional :lang/get keywords in the text map, for new errors + form inputs

To do this, the following major refactors were made:

• The reaction edit buffer spec now stores conditions as a vector instead of a map; this ensures stability when editing condition names
• Major changes to the dropdown code, including removing code unused in lrs-admin-ui and adding the new numeric path segment input