Feature: allow using of predefined code for dev link and user link

It would be useful if user could define dev URL and user URL once (in dedicated fields) and in project description those values could be called by using tags.

I.e.
Dev URL: https://github.com/codeforcroatia/legal-imamopravoznati
User URL: http://legal.imamopravoznati.org/

In Description when mentioning Dev URL, or User URL, it can be used one of following tags: {laddr_dev_url} {laddr_user_url}.

Example:
`Please contribute on [Github repository]({laddr_dev_url}).`

So in case URL would be changed, it needs to be updated only in one place - in dedicated Dev URL field, and all other mentions od that link in Description would use new URL automatically.

I would find it beneficial.

Start by locating the project description handling and the existing settings or form for project URLs. Trace how description text is rendered, then verify that dedicated Dev URL and User URL values can be referenced by {laddr_dev_url} and {laddr_user_url}; done means both tags resolve to their configured URLs and changed values appear wherever the tags are used.

Written by the indexing model from the issue text.