Make popups auto-scale with window scaling

[zee-bit]

Currently, when ZT is opened in a terminal with a large enough size, the popups do scale with the terminal up to a minimum dimension as defined in #1005. But, when ZT is opened in a terminal with a small dimension initially, the popups don't autoscale if the terminal dimensions are expanded.

Steps to reproduce:

• Set the terminal dimension to, lets say (85 x 35), then open any popup. (e.g. Help)
• On increasing the terminal's dimension, the popup dimension will remain unchanged.

This indicates, in general, the inability to handle small terminal sizes(related to #1005), and we'd definitely want it working on a range of sizes. Related discussion and potential solutions can be found in #zulip terminal > Small terminal sizes, or more specifically after this message.