Prevent confusion around filter conditions being implicitly joined by AND and filters joined by OR

[jhrcek]

People are often creating filters that are too restrictive (filter-out everything), because:

• they don't realize that multiple conditions within a filter are connected by conjunction (AND)
• it's "too easy" to add multiple condition to one filter so people create one filter with multiple condition where they really should be creating multiple filters.

We should take some steps to prevent this confusion. Two ideas to prevent the above mentioned problems:

• When rendering the condition preview, replace the ';' joining the condition with "a zároveň " possibly highlighted in red to make the connective more explicit.
• replace "checkbox-based" enablement of condition by "Přidat podmínku" button and dropdown to select the condition.

[jhrcek]

@triceo I implemented first improved prototype of conditions creation as described in the first comment ^ and shown in this screenshot:

You can play with the modified version in my testing deployment. I realize I'm about half-way to the solution you envisioned, but let's discuss the current improvement in person before I invest more time in it :-)

PS.: I made a couple of other changes:

• fixed regression introduced by migration to bootstrap 4 (https://github.com/RoboZonky/natural-strategy-setup/issues/8)
• changed the behavior of the accordion by enabling at most one section to be opened at time (thus less scrolling is required when editting the strategy).

[jhrcek]

Feedback from in-person discussion with @triceo Let's leave this as for now and see if people still find this confusing. Just include these minor modifications:

1. also make the "disjunction" that connects individual filters explicit - it would be separate div "joining" the individual filter divs in the preview.
2. change the styling of the connectives: use bootstrap pill badges - secondary for "or" and info for "and".
3. (unrelated) change "robot má udržovat prázdné portfolio." to mnou definované