Enhancement: render fourth level containers

[ctgraham]

Container rendering currently stops at the third level:

islandora_solution_pack_manuscript/transforms/ead_to_html.xslt

Line 48 in c0fee29

<xsl:template match="ead:c | ead:c01 | ead:c02 | ead:c03">

Additional levels could be helpful, such as a four-level:

• Subgroup
• Series
• Subseries
• File