Keep draft 'reason' when editing own draft

[smichel17]

Fixes the following bug:

• Start editing an installer
• Write something in the 'Reason' field
• Save a draft

Expected: All fields stay the same
Actual: 'Reason' field gets reset to empty, so I need to re-type it before submitting

At least one person has been re-writing the reason every time they save a draft (https://discordapp.com/channels/512538904872747018/546612632027922452/721809136232431647), and I'm sure there's others who submit with blank reasons, not realizing it disappeared.

This behavior was introduced with 2ecfe2c, which was itself a bugfix for #167, so we need to take care not to reintroduce it.

This is a draft because:

• It's kind of ugly code. I'd like to try and find a more elegant way to rearrange things.
• I have not tested it. I'd like to get the lutris site running locally and do this.
• Bad commit message. I'd like to clean up this PR text and include it there.

I haven't done these because I need to get to work 😆; I'll get back to this later (this weekend?), unless someone else wants to pick it up.