Refactor texter assignment campaign builder tabs

[ajohn25]

Is your feature request related to a problem? Please describe.
The Teams / Texters / Autoassign Mode tabs on the campaign builder all handle various aspects of assigning texts to texters

Describe the solution you'd like
A unified assignment tab which allows admins to select from any of these assignment flows as they desire.

Something like:

• If individual texters are assigned, split texts btwn them (as the Texters tab currently does)
• If teams are assigned, enable autoassign for the campaign and assign that team. Include "General" as a team so that turning on General autoassign works the same way as turning on a specific team.
• The "release replies" toggle from the Autoassign Mode tab should move to this unified tab as well

Describe alternatives you've considered

• Separate refactors of each of the tabs

Additional context
(design pass suggested)

• Should autoassign be renamed considering texters have to request texts manually even if autoassign is enabled? Something like “allow texters to request texts?”
• Should we only allow selecting either teams (+general) or individual texters? This would prevent conflict btwn the two distinct assignment flows