Allow editing unsent messages

add avatar changed event support issue #550

As of 0db4d71e (Prevent edits of
unsent messages), messages that are edits of (or replies to) unsent
messages were not allowed. This change was made because otherwise
the edits were discarded due to use of txnid rather than mxid in the
"m.relates_to" object. Remove this restriction and fix the issue by
replacing txnid with mxid in all related events when the message is
sent (and we obtain mxid from the server).

Currently translated at 62.7% (308 of 491 strings)

Co-authored-by: Tirifto <tirifto@posteo.cz>
Translate-URL: https://weblate.nheko.im/projects/nheko/nheko-master/eo/
Translation: Nheko/nheko

Currently translated at 61.9% (304 of 491 strings)

Co-authored-by: Tirifto <tirifto@posteo.cz>
Translate-URL: https://weblate.nheko.im/projects/nheko/nheko-master/eo/
Translation: Nheko/nheko

Currently translated at 63.7% (313 of 491 strings)

Translated using Weblate (Italian)

Currently translated at 63.7% (313 of 491 strings)

Co-authored-by: Daniele Scasciafratte <mte90net@gmail.com>
Co-authored-by: Quanterxotur <ganno.lexis.ag@mailbox.org>
Translate-URL: https://weblate.nheko.im/projects/nheko/nheko-master/it/
Translation: Nheko/nheko

Currently translated at 60.8% (299 of 491 strings)

Co-authored-by: Tirifto <tirifto@posteo.cz>
Translate-URL: https://weblate.nheko.im/projects/nheko/nheko-master/eo/
Translation: Nheko/nheko

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