Migrate icons to material design icons

[GretaD]

Ref nextcloud/groupware#38
To do

• All the Loading icons
• some leftovers (icons on attachments)
• change button to buttonvue - for the icon parts
• Requires Upgrade to nc/vue6beta3 #7060
• Requires 6.0.0-beta.4 nextcloud-libraries/nextcloud-vue#3092 because Fix Modal closing nextcloud-libraries/nextcloud-vue#3066

there are some bugs because of nc/vue update but not connected to this pr, those bugs can be found here: #7071

test carefully composer(send email, drafts and stuff) and loading envelopes

[st3iny]

I discovered multiple issues while testing.

NOTE: Some of those issues might be on main too or caused by the nc-vue upgrade to 6.0.0.

1. Send later ActionButtons are jumping on hover

This is most likely caused by the nc-vue upgrade.

Peek.2022-08-16.15-20.webm

2. Attachment modal can't be interacted with and closed

(I'm spamming left click as I hover various elements in the video.)

Peek.2022-08-16.15-29.webm

The close button is not working which resulted in me being stuck and having to force reload the page.

3. Account settings modal can't be closed via the X

However, it can be closed by clicking outside.

4. The account creation modal is missing its form

[GretaD]

the problems Richard found are fixed. This pr #7060 is needed to move on