Limit the ordering of the issues to the scope of their priority level

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Description

**Problem**
The prioritized labels don't have any effect anymore since the manual ordering of the issues has been added to the board. This can easily lead to inconsistent ordering.
**Example**
Let's assume we have three prioritized labels in this order: - high - medium - low
And let's assume the situation is this one: - Issue A (High) - Issue B (High) - Issue C (High)

- Issue D (Medium) - Issue E (Medium) - Issue F (Medium)

- Issue G (Low) - Issue H (Low) - Issue I (Low)
With the current behaviour of the sorting (which is not automatic at all anymore) we can easily end up like this:

• Issue E (Medium)
• Issue D (Medium)
• Issue F (Medium)
  
  
• Issue A (High)
• Issue B (High)
• Issue C (High)
  
  
• Issue G (Low)
• Issue H (Low)
• Issue I (Low)

Or even like this:

• Issue E (Medium)
• Issue G (Low)
• Issue B (High)
  
  
• Issue A (High)
• Issue D (Medium)
• Issue C (High)
  
  
• Issue I (Low)
• Issue F (Medium)
• Issue H (Low)

Which doesn't make a lot of sense and is really messy. It would be better to have something like this (based on the first example):

• Issue A (High)
• Issue B (High)
• Issue C (High)
  
  
• Issue E (Medium)
• Issue D (Medium)
• Issue F (Medium)
  
  
• Issue G (Low)
• Issue H (Low)
• Issue I (Low)

Or like this (based on the second example):

• Issue A (High)
• Issue B (High)
• Issue C (High)
  
  
• Issue E (Medium)
• Issue D (Medium)
• Issue F (Medium)
  
  
• Issue G (Low)
• Issue I (Low)
• Issue H (Low)

Meaning that we can manually reorder the issues but limited to their level of priority. Otherwise the prioritized labels are totally useless and can be replaced by regular labels (in other words prioritized labels are useless with the current behaviour).
### Proposal

Users should be able to manually order the issues, but not out of the scope of their priority level.
Issues with no priority should be treated as the lowest priority possible, meaning lower than the lowest prioritized label.

Links / references

Original proposal: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9770#note_28613498
Let's not forget how to deal with multiple prioritized labels to avoid a regression: https://gitlab.com/gitlab-org/gitlab-ce/issues/18554

Documentation blurb

The GitLab manual is still describing the old behaviour (as it should still be): https://docs.gitlab.com/ce/user/project/labels.html#prioritize-labels

~"feature proposal"