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.

首先定位项目描述的处理逻辑，以及现有的项目 URL 设置或表单。跟踪描述文本的渲染方式，然后验证是否可以通过 {laddr_dev_url} 和 {laddr_user_url} 引用专用的 Dev URL 和 User URL 值；完成的标准是两个标签都解析为其配置的 URL，并且更改后的值会出现在标签使用的所有位置。

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