Fix mobile MetaMask deeplink and polish steward review UI

[JoaquinBN]

Summary

• Drops encodeURIComponent on the MetaMask dapp path and switches to link.metamask.io; mobile MetaMask was opening its in-app browser to a blank tab because the encoded /, ?, # produced a malformed destination URL. Also adds a Coinbase Wallet deep link (go.cb-w.com/dapp?cb_url=...).
• Renders contribution type descriptions as markdown on the type detail page, preserves line breaks, clamps to six lines by default, and shows a Show more / Show less toggle only when the content actually overflows.
• Surfaces each user's linked GitHub / X / Discord accounts as compact brand pills next to the user name on the steward submissions view; backend StewardSubmissionSerializer.get_user_details now always merges the three social connections into user_details and the queryset adds matching select_related to avoid N+1.

Test plan

• On a real phone with MetaMask Mobile installed, tap Connect → MetaMask and verify the in-app browser actually loads the dapp (including hash routes) instead of a blank tab.
• Open a contribution type detail page whose description contains markdown / multiple paragraphs and confirm it renders formatted, clamps to ~6 lines, and the Show more / Show less toggle works.
• Visit Stewards → Submissions and confirm GitHub / X / Discord pills appear next to users who have those accounts linked, with the GitHub and X pills linking to their external profile.