Improve UI of create/edit form

[felixhahnweilheim]

PR for #392

• moves the location field after the checkboxes "all day" and "recurring" -> so they are directly under date and time
• makes date and time fields half width also on small screens (saves a lot of space on mobile)
• the Time Zone always appears under the end time (also on mobile)

Screenshots before and after:

[luke-]

@felixhahnweilheim Thank you!

@Eladnarlea What do you think? Maybe we should create another issue for the editor under mobile?

[felixhahnweilheim]

Just to make it clear, this PR does not affect the editor and the Description field.
(I just did the screenshots on different platforms (1.15 and 1.14) that's why there is a difference.)

[luke-]

@felixhahnweilheim Yes, thats clear. I've just merged your PR. Please also modify the CHANGELOG. I've added this change: d0070cf