Add symbols for all supported narrows

[neiljp]

This was originally noted as part of #1348, but deserves splitting out.

Very early in the history of the project, there were markers/icons/emoji for narrows, eg. for all messages there was a 'house' symbol, since at the time this was the 'home' in the web app.

These symbols were subsequently removed, since it is difficult to find unicode codepoints which are consistently viewable across terminals, such that running with a default install doesn't elicit "this doesn't look right" from a wide range of users.

This issue is concerned with considering the reintroduction of symbols, but for all the narrows which we currently support.

Originally symbols were only shown as prefixes in the top-left part of the application, and we'd likely want to include them there for consistency, which may lead to us increasing the width of the left panel slightly.

It would also be useful to show these symbols as prefixes to each narrow in the line below the "Messages" title line.

As a first step this will need identification of suitable symbols for each narrow; these include:

• All messages (via a)
• All direct messages (via P)
• All stream messages (new narrow to be added to the left side, likely via S)
• Direct message conversation (via z) [*]
• Group direct message conversation (via z) [*]
• <stream-name> (ie. stream narrow) (via z) [*]
• <stream-name> > <topic-name> (ie. topic narrow) (via z) [*]
• (All) mentions (via #)
• Starred messages (via f)

Note that in line with the original challenge - and why old symbols were removed - any new proposed symbols will need checking for compatibility on 'common' terminals/fonts.

Those marked with [*] have an existing prefix, and can be an extension of the work on #1348 / #1386. However, we'll likely want to introduce a set of symbols at once in order to reduce confusion, so please discuss this in the stream before implementing - see #zulip-terminal > Narrow symbols #T1420.

Connections to other issues:

• #1343 - For topic narrows, prefixing <stream-name>: topic narrow is not likely useful; similarly for direct message 'titles' like Direct message conversation, which #1343 would enable dropping.
• #1197 - This work will involve adding the 'All stream messages' narrow.
• #938 - If this was supported, we'd have fewer constraints on symbol choice, though we'd likely still aim for a commonly-viewable theme in addition to an 'emoji' and 'super basic' one.

[neiljp]

I'm closing this now, since it is sufficiently complete; for followup please discuss in the zulip-terminal stream on chat.zulip.org.