V8.7RC Block list adjustments

[bjarnef]

Prerequisites

• I have added steps to test this contribution in the description below

Description

While working on a solution for #8689 I noticed various issues, which I have fixed in this PR.

• Accessibility enhancements.
• Added rel="noopener" for security reasons.
• Stored path views/propertyeditors/blocklist/blocklistentryeditors/ in a variable.
• Added properties to vm.labels after localization.
• The add/plus button didn't had same width and height. Furthermore it looked like you never hit the button because it is moving when the cursor position change. Instead this is updated in transform property where it is vertically centered and moved 80% to the left, so the cursor doesn't cover the icon, but it still feels like you hit the button.

[bjarnef]

@nielslyngsoe this seems to be a bit nicer. Futhermore maybe umb-code-snippet component could be used in the unsupportedblock editor?
https://github.com/umbraco/Umbraco-CMS/blob/v8/8.7/src/Umbraco.Web.UI.Client/src/views/propertyeditors/blocklist/blocklistentryeditors/unsupportedblock/unsupportedblock.editor.html#L11

E.g. like this change for v8.8?
#8548

[nul800sebastiaan]

Wonderful updates @bjarnef - thanks very much! 👍