Get `UPLOAD_FILE_MAX_SIZE` setting from the backend

[kdmccormick]

This was reported here: https://discuss.openedx.org/t/upload-size-limit-not-working-platform-wide/18610/4

It's currently hard-coded to 20MB:

frontend-app-authoring/src/constants.ts

Line 56 in 0c59b78

export const UPLOAD_FILE_MAX_SIZE = 20 * 1024 * 1024; // 100mb

There's a setting on the CMS backend which can be adjusted, called MAX_ASSET_UPLOAD_FILE_SIZE_IN_MB: https://github.com/openedx/openedx-platform/blob/919a479dac43701fb1bbd27a9e3a9bcd2d85b11c/cms/envs/common.py#L935

In openedx-platform, this setting should be piped into one of the existing REST APIs which frontend-app-authoring calls. Then, in frontend-app-authoring, that setting's value should be used as the max instead. The UPLOAD_FILE_MAX_SIZE frontend constant should be deleted.

[kdmccormick]

This would be a good candidate to backport to Ulmo.

[bradenmacdonald]

Relates to / duplicates #1661 ?

[kdmccormick]

Yes, a dupe indeed :)