link_utils: exposing whitelisted protocols to user settings

[shubham-padia]

Fixes #1284.
Adding config option to set protocols in the config that are whitelisted to be opened directly.
The behaviour is documented in docs\howto\customize-link-protocols.md.

Supersedes #1401. Keeps the original author same.

Fixes:

Screenshots and screen captures:

Platforms this PR was tested on:

• Windows
• macOS
• Linux (specify distro)

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
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Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[alya]

We should be using standard Zulip capitalization throughout (sentence case).

[shubham-padia]

Resolved!

[alya]

Suggested change

	Note: Each protocol should include the trailing colon (:), e.g., "mailto:" instead of "mailto".
	Note: Each protocol should include the trailing `:`, e.g., "mailto:" instead of "mailto".

[shubham-padia]

Resolved!

[alya]

@andersk Tagging for your review.

[andersk]

getConfigItem writes the default value into the config file if it’s missing, so if we ever add new default protocols they’ll never be visible to someone who’s ever launched an old version.

(Was this a bad design decision for getConfigItem? Yes.)

[shubham-padia]

I have added getConfigItemWithoutSettingDefault for now, will do a followup PR to make getConfigItem to do the same after checking every use of getConfigItem and making sure there are no behavioural changes introduced by doing so.

[shubham-padia]

Made changes requested!