(dev/release#17) Extension UI - Show developmental icon for alpha/beta-stage extensions

[totten]

Overview

Each version of an extensions is published with two fields, <version> (e.g. 1.1) and <develStage> (e.g. alpha).
Confusingly, the two fields sometimes convey redundant information - but not always. A few cases:

• (A) Sometimes a <version> as 1.0-alpha1, 1.2.beta2, or 2.3dev. This would make sense if the
  developer does formal tagging/releasing for developmental versions. (In this case, you don't really need the
  icon - because the version-number tells you.)

• (B) Other times, you might have a <version> which simply says 1.0 or 2.0 -- and then supplemental
  information where the <develStage> says alpha or beta. This could make sense if the developmental
  version is being continuously updated without formal tags/releases.

• (C) Under dev/release#17, we have another case -- where the core-extensions have version#s which match the
  core version# (because they are released together), but the devel-stage of the extension is only
  alpha/beta (because the extension is still evolving/optional/not-fully-supported).

Before

The "Development Stage" is not specifically indicated. This is OK for (A) but not (B) or (C).

After

There's an icon in the table to signal the development stage. So the stage is indicated in all cases (A,B,C).

[civibot]

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[colemanw]

This looks fine. I made a couple tweaks to the markup and css, and avoided going down the rabbit-hole of further improvements to that ancient extension management screen. I think there was some effort being made to rewrite that screen completely, which would be wonderful.