Add 'Schedule' buttons in the Jobs page

[my8100]

This PR adds a schedule endpoint based on #256.
Note that the 'Cancel' and the 'Schedule' buttons still not visible
in Python 3 due to #312.

[codecov]

Codecov Report

Merging #321 into master will decrease coverage by 0.45%.
The diff coverage is 11.11%.

@@            Coverage Diff             @@
##           master     #321      +/-   ##
==========================================
- Coverage   68.37%   67.91%   -0.46%     
==========================================
  Files          16       16              
  Lines         819      826       +7     
  Branches       96       98       +2     
==========================================
+ Hits          560      561       +1     
- Misses        230      236       +6     
  Partials       29       29

Impacted Files	Coverage Δ
scrapyd/website.py	56.43% <11.11%> (-3.14%)	⬇️

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[codecov]

Codecov Report

Merging #321 into master will decrease coverage by 0.45%.
The diff coverage is 11.11%.

@@            Coverage Diff             @@
##           master     #321      +/-   ##
==========================================
- Coverage   68.37%   67.91%   -0.46%     
==========================================
  Files          16       16              
  Lines         819      826       +7     
  Branches       96       98       +2     
==========================================
+ Hits          560      561       +1     
- Misses        230      236       +6     
  Partials       29       29

Impacted Files	Coverage Δ
scrapyd/website.py	56.43% <11.11%> (-3.14%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5298f9b...c0a9b06. Read the comment docs.

[my8100]

@Digenis, What do you think about the added 'Schedule' button?

[Digenis]

There's no way to add arguments and settings
and, taken from the finished jobs,
it creates the confusion that the job will run with the same arguments.

The reason I wrote the cancel shortcut is that the solution suits any user.

If we add a form to add arguments, we are getting in to the scope of other tools
such as scrapyweb. What do you think?

[my8100]

Not every spider run needs arguments and settings from cmd.
And it's convenient for repeating crawling jobs with just a click.

PS: typo for scrapydweb

[jpmckinney]

I share the concerns of @Digenis in #321 (comment)

Edit: Also, it's a bit odd to only be able to schedule spiders that have finished jobs. I would expect the UI to then also allow scheduling any spider in any project. Better not to create that user expectation, if we don't intend to fulfill it.