Stripping disallowed attributes by (insert|delete)Content #1088

Simple scenario.

1. Copy a paragraph with italic and link.
2. Paste it to the Heading 1. Inserted text should be stripped
3. Paste it to the Heading 2. Inserted text should be a link only.
4. Paste it to paragraph. Inserted text should not be stripped.

Simple scenario (element).

1. Copy image.
2. Paste it to the editor. Image should be inserted with an alternative text "Sample image".

Nested nodes.

1. Copy a list item with bold and link.
2. Paste it into the empty block (directly to the root) . Inserted list item should be a bold link.
3. Paste it into the empty block in BlockQuote. Inserted list item should be a bold only.

Auto paragraphing.

1. Copy a text with bold.
2. Select all content in the editor.
3. Paste copied text. Inserted content should be a paragraph and should be stripped from bold.

Auto paragraphing (disallowed block).

1. Copy Heading 3 with bold and italic.
2. Select all content in the editor.
3. Paste copied text. Inserted content should be a paragraph and should be stripped from bold.