[FIX] Linear regression: Fix Elastic net; Fix Auto-apply buttons

[janezd]

• Elastic net were mixed L1 and L2 in the opposite proportions(!).
• The related slider used to be in the wrong part; while this was already fixed a few weeks ago, the code for constructing the slider was still in add_bottom_buttons.
• The widget did not have the auto-apply button and would fail the base learner widget tests; this was cause by the redundant method add_bottom_buttons.
• The widget did not have any tests (I assume that it's because it'd fail them)

I added a test to the base test class for learner widgets, which checks whether the class has unconditional_apply method. This method is be required for proper implementation of force-apply. If the widget intentionally removes it, it will have to override this test (or avoid being tested).

[codecov-io]

Current coverage is 88.67% (diff: 100%)

Merging #1601 into master will not change coverage

@@             master      #1601   diff @@
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  Files            78         78          
  Lines          8134       8134          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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  Hits           7213       7213          
  Misses          921        921          
  Partials          0          0          

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[BlazZupan]

Checked everything, works as expected now.