Fixes frontend to properly call invitation accept endpoint.

[rtibbles]

Summary

Description of the change(s) you made

Fixes oversight from #4615 where the frontend was not properly updated to use the new invitation accept API endpoint for synchronous acceptance of channel invitations

Manual verification steps performed

1. Invite another user to a channel
2. Accept that invitation as the other user
3. Rejoice!

References

Fixes #4643

Comments

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Contributor's Checklist

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Reviewer's Checklist

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[marcellamaki]

lgtm - changes from patch which was causing an error, and manual QA test confirms that i was able to accept the invitation and open the channel

[radinamatic]

I was also able to accept the invitation and open the channel, but the invitation remained sticky on the top of My channels page even after clicking the ✔️ button several times, navigating away and back, and even after signing out and in again... I eventually had to decline the invitation (that I previously accepted!), in order to make it go away finally 🤷🏽

[akolson]

cc @rtibbles