Allow submitting assignments with attached files

Create assignments with different configuration options: Only online text (with or without word count), only files (with limit), both. Require students click submit button Require that students accept the submission statement Attempts reopened Group submissions To test: Check that you cannot add/edit a submission if it uses online text and MDL-55113 isn't applied to your Moodle. Check that you cannot submit for grading if you require to accept submission statement (will be fixed in Moodle 3.2). Otherwise, check that you can add/edit a submission: online text and/or files. Check that the edit and submit buttons are shown and with the same text as in Moodle web for each different status. Check that you cannot add more attachments than allowed. Check that, if you configure a size limit different than activity default, you cannot select files bigger than that size. If the assign doesn't allow drafts, check that saving a submission already sends it to grading. Otherwise, check that you can see a button to submit it for grading. If the assignment allows more attempts, check that you can see buttons to create a new attempt or copy the previous. Check that both of them work as expected.