Rewrite description for URL alias on node/add/*

Problem/Motivation

Adding a slash at end of a URL alias in "Add Content" option results in page not found error at the url.

Steps to reproduce the bug -
1. goto Add Content, fill in the required fields
2. scroll down to find URL settings tab. There they have already mentioned not to use a trailing slash for aliases, but to produce this bug you should type in 'somename/'
3. Now save changes. It will redirect you to the alias, but you'll get the page not found error.

Screenshots : https://imgur.com/a/hruq3
The same are attached as files as well

Proposed resolution

Since this problem is because browsers simply ignore a trailing slash in URLs, while our alias database won't know that.
Escape the trailing slash while storing the alias at backend, and/or alert user at frontend if entered a trailing slash.

Remaining tasks

None

User interface changes

None.

API changes

None

Data model changes

None