Add component for inline alerts

[matthew-white]

Filing a follow-up issue about work done in v2026.3:

• We now show a variety of alerts (errors/warnings) in the entity upload modal: Smooth out entity CSV upload experience #1784
• We also show an error in other modals where a property can be created: Wait to create actor properties in "create new app user/link" modal #2165
• These two sets of alerts are similarly styled, but it was just copy/paste. This issue is to add a component or HTML class to be shared between the two.
• The new component could be used for future inline alerts: Move error messages inline #1157