Change logos to OpenSTL and hide Hack for LA footer

[staceymck]

Theme config:
Store brigade details as constants in new theme config file to make switching between LA and STL branding more dynamic.

Footer:

• Only render the 'developed by Hack for LA' footer if brigade name constant is Hack for LA.
• Only run tests that check for the presence of this wording in the footer if the brigade name constant is Hack for LA.

Logos:

• Use prefixes to identify LA and STL logo files and update manifest.json and index.html to use the STL logos.
• Conditionally render logo in header across app based on brigade name constant from theme config file