Ensure consistent animation rate across expandable elements

[cmsetzer]

Before:

• animationSpeed was an attribute of LilHeader
• animationSpeed affected not only the LilHeader nav menu, but also Swup transitions and the +/- icon that displays when opening LilExpandable on the Careers page
• However, animationSpeed did not affect the actual expandable content in LilExpandable, opening up the potential for harmless but weird effects if the user clicked an expandable multiple times in quick succession

After:

• animationSpeed has been renamed to the more descriptive animationRate
• animationRate is now a global constant, not a property of LilHeader
• animationRate now affects all elements of LilExpandable consistently, preventing the aforementioned visual bugs
• Tweaked LilExpandable transition durations a bit to fit better with other elements

[bensteinberg]

(I'm going to take the liberty of merging this.)