This makes sure that when creating a report, the reportText field
contains a template text. Some of the heading blocks in the template
are readonly and can't be removed.
Note: it only works now on add, after save, on edit, the headings
are no longer readonly (still to be implemented).

This makes sure that if the richTextEditor contains h1 tags marked
with the mandatory class, they can be edited but the tags itself
can not be deleted. Also, when the content is saved, the resulting
html also still has the mandatory class for the h1 tag (this was not
the case before).

In order to be able to save richText content containing also a class,
we need to allow this in the HtmlPolicyBuilder, which is being used to
sanitize the HTML before saving. We updated this one to allow the
class attribute for h1 and p elements.

In the RichTextEditor, replace mandatory elements with a placeholder
when all their text is being deleted.

In the RichTextEditor, replace mandatory elements with a placeholder
when trying to delete their last content character.

Changed the style of the h1 mandatory element. Made sure the mandatory
elements are only having a specific style when in the RichTextEditor.

It is now possible to configure within the anet.yml, through
report.reportText.placeholder the html template to be used per
default for the report text.

This makes sure that if an unstyled block has the mandatory class,
it is also being saved as a html element with the mandatory class.

This makes sure that when specifying a default template for a
richTextEditor type of field, we can also define a placeholder for
the html elements having the class mandatory. This placeholder
is being used to prefill the element if this one becomes empty.
Both the class attribute and the placeholder attribute of an HTML
tag of type h1, h2, h3 or p are being persisted in the database. This
in order to be able to still know the mandatory elements and their
eventual placeholders when editing an object with rich text content.

…Agency/anet into GH-3081-richt-text-editor-templates

This makes sure that if the RichTextEditor is using a template containing
mandatory blocks, we can not merge them with next/previous blocks using
delete/backspace.

This makes sure that if the RichTextEditor is using a template containing
mandatory blocks, we can not merge them with next/previous blocks using
delete/backspace and we can not delete them.

Prevent return when:
- at the beginning of a mandatory block => it would result in an empty
  mandatory block and a new non-mandatory block containing the text of
  the block which used to be mandatory.
- the selection contains at least one mandatory block => it would result
  in deleting the selection including the mandatory block
- exactly one whole mandatory block is selected, it would result in the
  contents of the mandatory block being deleted and the addition of a
  new empty block afterwards.

Prevent delete or backspace when:
- the selection contains several blocks and at least one of them is
mandatory => we don't want to lose mandatory blocks

Prevent delete when:
- at the end of a block when the next block is => we don't want to merge
  a mandatory block into a previous one

Prevent backspace when:
- at the beginning of a mandatory block => we don't want to merge a
  mandatory block into a previous one

The reason we don't want to merge a mandatory block B into a previous
block A as:
- if block A is not mandatory, block B would lose it's style and data
  (thus also the mandatory property)
- if block A is also mandatory, we would end up with only one mandatory
  block instead of two.

Prevent pasting on a selection when:
- at least one of the selection blocks is mandatory. It would result in
  the removal of mandatory blocks.