[IMPROVEMENT] Improve performance of rendering alerts and silences

[KarboniteKream]

Description

This PR improves performance of rendering alerts and silences in Vue.

Currently, all alerts and silences are always rendered and hidden using the collapse CSS rule. This causes rendering delays on page load and whenever components need to be re-rendered (for example, on form input change; #370).

With changes introduced in this PR, all alerts and silences are now hidden behind v-if, so the HTML elements are not generated/rendered until the expression is truthy.

Detailed changes

• Introduce toggleComponent(), which dynamically toggles the state of components
  • The initial state of all components is falsy, so they're not shown by default
  • Since the components object does not have any properties, it needs to be updated using this.$set()
• Use toggleComponent() for all elements that previously used toggleTarget() and collapse
  • Add v-cloak to top-level element of specific components
• Replace v-show with v-if so we skip rendering of v-for if it's not necessary
• Remove unnecessary prevent modifier on @click
• Improve caching of computed methods
  • We can reuse filterActive{Alerts,Silences}
  • Since we return a Set, there's no need to sort the values
• Rename toggleTarget() to toggleCollapse()
  • Alerts and silences on the project detail page are expanded by default, so toggleComponent() cannot be used

Improvements

Before the improvements, the page would be mostly unresponsive until the Alerts block is rendered.
Note: All measurements are approximate page reload times without cache, and were performed with 1660 alerts, 25 silences and 6000 farms.

Component	Before	After
Home page	2s	0.8s
Global alerts section	2s	2.5s
Services page	5.2s	1.9s
Link Farm page	16s	6.7s

• Page loads are now completely responsive, except for Link Farm since that needs to render the HTML of 6000 farms.
• The global Alerts section, now takes 500ms longer to open, since the HTML elements need to be generated.
• Form input lag is almost nonexistent, unless the global Alerts section is open.
• The bottleneck now seem seem to be API responses

[KarboniteKream]

For the Link Farm page, we could improve the performance by limiting the number of items shown. Creating a separate API and controlling rendering in Vue would be one of the options.

[KarboniteKream]

Using v-for with v-if is not recommended. Additionally, this iterates over all active alerts/silences, which takes time, but currently doesn't look like a problem. I'll improve this in the next PR.

[KarboniteKream]

The behavior here has changed slightly. I noticed that filterActiveSilences was changed from state == 'active' to state != 'expired'. I think we want the same behavior here?