Feature: Hide phone numbers in conversation headers

[char]

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Description

This is a pull request that adds a 'Hide phone numbers in conversation headers' option to the preferences.

This solves #3561 and #1741.

[char]

I'm having some issues where sometimes the Preferences page fails to load. Can somebody help out? It doesn't seem to be consistent at all.

[scottnonnenberg-signal]

I'm going to close this, as we need to think very hard about any new option we add. We want to avoid complexity for our users wherever possible. As far as the original issue we're trying to address with these changes, we have other ideas about how to address it.

[abowerman]

Phone numbers should still be hidden in the UI by default.

RE: #3561 / #1741