feat(NewMessageUploadEditor): multiple files sharing support 🖼️+🖼️

[Antreesy]

☑️ Resolves

• Support sending and receiving a group of shared files in a single message
• Fix #…

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏚️ Before	🏡 After
B	A

🚧 Tasks

• Mobile clients support:
  • As per backend setup, if message with file share contains a caption, it will replace the default message text ({file}). So we need to manually add references to the message text (see Message.vue)
  • When multiple files are loaded, they'll get indexed references - file-1, file-2 (and file-missing-3 for files deleted afterwards)
  • When sending multiple files, we still need to upload it via apps/files_sharing/api/v1/shares one by one, but metadata object should contain 'noMessage: true' to nont generate a separate message for each file. Instead, we're sending a new request to apps/spreed/api/v1/chat/{token}/share-files with caption and share ids of all the uploaded files
• Autotests
• Capabilities
• Silent sending - check if possible
• User bubbles - don't render for prefilled messages
• Manual testing
• Visual check
• Code review

🏁 Checklist

• 🌏 Tested with Chrome, Firefox and Safari or should not be risky to browser differences
• 🖌️ Design was reviewed, approved or inspired by the design team
• ⛑️ Tests are included or not possible
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required

[jancborchardt]

Looks super nice generally! Some feedback:

• In the modal, the "Send" button is now not primary anymore, would be cool if the "Paper plane" icon button would be a primary button icon+text
• Currently when composing, the images are above the text, but when sending the text comes first. In other messengers, when you send a message with a picture, usually the picture is above the text too.
• Sort of related to the previous point, maybe it would be good to put the photos next to each other or tile them somehow else when there are multiple ones. Otherwise the text and photos are very far apart and the message also takes a lot of space if you send a message with 10 pictures

[Antreesy]

Rebased onto master, dropped all irrelevant/outdated/conflicted code