repo: able fill pull request title by template from md file

[ivanbogomoloff]

Hello.
I migrate from bitbacket to gogs, and in PR i want to be able auto fill PR title field.
So i write some simple code for this feature and my explanation how it works.

It works like .gogs/PULL_REQUEST.md
But fills pull request title field.
And have some feature, like branch placeholders

For example init repository:

1. Init repo with PULL_REQUEST_TITLE.md

$ ... git init etc...
$ echo 'Merge {{headBranch}} into {{baseBranch}}' > .gogs/PULL_REQUEST_TITLE.md
$ git add . && git commit -m "custom pr title for gogs" && git push origin master

2. Then create new branch for test compare web form

$ git checkout -b test-1
$ echo 'changes in readme' > README.md
$ git add && git commit -m "changes in readme" && git push origin test-1

3. Go to link http://localhost:3000/ivanb/test1/compare/master...ivanb:test-1
   And we will get some html code

...
<div class="ui segment content">
<div class="field">
   <input name="title" placeholder="Заголовок" value="Merge test-1 into master" tabindex="3" autofocus="" required="">
</div>
...

Thanks for attention