Resolve "Script trains"

Closes #72 (closed) #91 (closed)

What does this MR do (summary)?

• Adds script train functionality back
• Adds an API for scripts to check submissions
• Makes feedback only visible to the appropriate roles

Does this MR meet the acceptance criteria?

• The main application was run to make sure the server still runs without errors.
• Tests were written to verify the behaviour of this code.
• Javadoc and other comments were added to make the code understandable.
• The code adheres to the style guidelines.

docs/ScriptApi.md

+# Script API
+
+Submit can be configured to send student submissions to scripts to be automatically graded.
+Anyone can create one of these scripts using the script API.
+
+### Receiving data from Submit
+Submit sends the following information:
+
+##### Multipart Form Data 
+| field | type | note |
+| ----- | ---- | ---- |
+| file | MultiPartFile | |
+| submissionId | Long | |
+| scriptId | Long | |
+| key | String | 512 byte unique key, valid for one hour, can be used to send multiple feedbacks |
+| grade | Boolean | True if the script is allowed to send grades |
+
+#### Sending feedback to Submit
+Submit accepts feedback in the following format:
+
+Endpoint: `/script/feedback`
+
+##### JSON
+| field | type | note |
+| ----- | ---- | ---- |
+| submissionId | Long | Needs to match with the id sent |
+| scriptId | Long | Needs to match with the id sent |
+| key | String | Needs to match with the key sent |
+| visibleFor | String | STUDENT, HEAD_TA, TA, or TEACHER, will be visible for specified role and above |
+| textualFeedback | String | |
+| scoreType | String | Applies only when allowed to grade, see below for allowed values |
+| score | Int | Applies only when allowed to grade, see below for allowed scores |
+
+##### Allowed scores
+| type | range | note |
+| ---- | ----- | ---- |
+| PASS_FAIL | 0, 1 | 0 = FAIL, 1 = PASS |
+| DUTCH_GRADE | 10 - 100 | 10 = 1.0, 100 = 10.0 |
+| DUTCH_GRADE_1000 | 100 - 1000 | 100 = 1.00, 1000 = 10.00 |
+| SCORE_OUT_OF_10 | 0 - 10 | |
+| SCORE_OUT_OF_100 | 0 - 100 | |
+| SCORE_OUT_OF_1000 | 0 - 1000 | |
+| SCORE_OUT_OF_10000 | 0 - 10000 | |
+| UNLIMITED | 0 - 2,147,483,647 | |
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