feat: Updates salesforce opportunity line item form validation

[brobro10000]

Adds validation on the SubscriptionPlanForm class for salesforce_opportunity_line_item
Uses the approach of a form validation as opposed to model validation to account for existing salesforce_opportunity_line_item that do not match the following format:
Regex: /^00k[a-zA-Z0-9]{15}$/
18 alphanumeric characters where the first three characters must be the sequence '00k'
Only the lower case 'k' will be accepted

Overrides the help_text for the field to inform the user of the new format restrictions.
Adds additional text during validation error specifying that the salesforce_opportunity_line_item must start with '00k'
Updates the seeded data for devstack with the correct format.
Updates get_random_salesforce_id function to match the format due to its uses of generating random salesforce_opportunity_line_item field values for tests

Description

Description of changes made

Link to the associated ticket: https://openedx.atlassian.net/browse/ENT-XXXX

Testing considerations

• Include instructions for any required manual tests, and any manual testing that has
  already been performed.
• Include unit and a11y tests as appropriate
• Consider performance issues.
• Check that Database migrations are backwards-compatible

Post-review

Squash commits into discrete sets of changes

[iloveagent57]

nit: you can use parens to format this type of thing across lines without using backslash, i.e.

if (
    product.plan_type.sf_id_required
    and self.cleaned_data.get('salesforce_opportunity_line_item') is None
    or not verify_salesforce_opportunity_product_line_item(self.cleaned_data.get('salesforce_opportunity_line_item')
):