Template creation support for excalidraw files

[juliushaertl]

• Requires #6

• We need to implement a TemplateFileCreator to let the files app know that it should offer creating new files from tempaltes for the excalidraw mimetype.

https://github.com/nextcloud/text/pull/5717/files#diff-7bcfa8fd8ed17d9329880f10eb0924e5ea2d615a02c9a31b2fd9a02fab7b594eR52-R59

Related PR on doing it more lazy https://github.com/nextcloud/server/pull/44952 https://github.com/nextcloud/text/pull/5717

[juliushaertl]

Already done as part of #16

Filed https://github.com/nextcloud-gmbh/whiteboard/issues/22 for some future improvement