Indicate web-public streams differently

[neiljp]

What does this PR do?

This is intended to fix most of #712 (#1135)

See #zulip-terminal > Web public marker #T1135/#T712

Tested?

• Manually
• Existing tests (adapted, if necessary)
• New tests added (for any new behavior)
• Passed linting & tests (each commit)

Commit flow

• Add model accessor for stream type, given stream id
• Use new accessor outside of model (in UI)
• Add fixtures for a web public stream & adjust existing tests
• Add symbol and extend existing code minimally to indicate symbol in most cases
• Add symbol rendering in stream list
• Refactor duplicated if/elif blocks to use new common dict in ui_mappings (descriptions)
• Refactor if/elif blocks to use same dict for icons too

Notes & Questions

This needs some tests fixed and I'm planning an additional refactor, but pushing now to get feedback.

update Tests are now fixed and extended.

Visual changes

Currently, web-public streams are shown as A. This is not ideal, but I've not found a reliable symbol/character yet (W in bold was very full/blocky). If you fetch the PR you can play with variations by changing the character in symbols.py.

update After debate on the stream, we currently have ⊚ as an approximation to a narrow (1-character-wide) circular symbol. We can explore this further when we explore symbol themes #938 (maybe #1114)

[neiljp]

Merging this is temporarily blocked on reports that the selected symbol doesn't render in WSL, though does in Windows Terminal.